This invention relates to ignition timing control systems and particularly to an ignition timing control system having an electronic distributor.
The ignition timing is generally determined by the load and the engine speed. The ignition timing is calculated from the output of the detector for the intake air flow representing the value of the load and from the output of the crank angle sensor associated with the engine speed, and an ignition signal is produced by triggering the output of a DRef signal (which occurs the cylinder-count times each time the engine rotates twice) produced each time each cylinder reaches a predetermined crank angle, thereby controlling the ignition in each cylinder. When a previously specified cylinder reaches a certain crank angle position, a CPU within the ignition control system generates a Mark-A marked signal for specifying a cylinder to be ignited. When the Mark-A marked signal is determined, the order of cylinders to be ignited next is naturally determined. The electronic distributor responds to the Mark-A marked signal to supply an ignition signal to the ignitor of the cylinder specified by that signal, and a next ignition signal to the ignitor of another cylinder in a predetermined order.
Therefore, the ignition timing control system has a sensor provided to produce a signal (Mark-signal produced at intervals of a predetermined angle each time the engine rotates twice) when the engine reaches a crank angle at which a particular cylinder is in a predetermined piston position. The Mark-A signal is supplied to the electronic distributor on the basis of this Mark signal.
However, if the Mark signal is not produced due to the failure of the sensor, the ignition signal can not be normally distributed to each cylinder when the output of the reference cylinder signal Mark-A becomes abnormal.
Thus, as is disclosed in the Japanese Patent Application Gazette No. JP-A-62-225770 filed by Hitachi, Ltd. on Mar. 28 in 1986, signal generating means for backup is used. In other words, when the Mark signal is detected to be abnormal, the output signal from the backup signal generator is used in place of the Mark signal so that an ignition signal is generated and distributed to each cylinder.
This prior art requires additional signal generating means for backup of the main sensor in case of the abnormal state of the Mark signal, thus increasing the cost of the whole ignition timing control system.